1. Field of the Invention
The present invention relates to a silicon carbide semiconductor device and a method of manufacturing a semiconductor device.
2. Description of the Related Art
A silicon carbide (SiC) semiconductor device typically has a high breakdown field strength and can therefore control a high current. Thus, the SiC semiconductor device can be used for controlling a motor of a hybrid vehicle, for example.
In order to increase electric current that flows in a semiconductor device, a channel density can be increased. In a silicon semiconductor device, a metal-oxide semiconductor field-effect transistor (MOSFET) having a trench gate structure is in practical use. When a trench gate structure is applied to the SiC semiconductor device, difficulty arises.
A thickness, that is, a depth of a base region is sum of a thickness of a source region and a length of a channel. Thus, if the thickness of the base region is reduced, a channel resistance can be reduced. A drain-source breakdown voltage depends on the thickness and an impurity concentration of the base region. A portion of the source region located at an edge of a trench may be slightly lost during a process for adjusting a shape of the trench. Thus, the source region is required to have a sufficient thickness in case of the loss, and the thickness of the base region is set taking into consideration such circumstance.
Therefore, conventionally, the thickness and the impurity concentration of the base region are set so that the source region has a sufficient thickness, the channel resistance is low, and a high drain-source breakdown voltage is secured as described, for example, in JP-2007-281265-A.
If the impurity concentration of the base region is reduced for securing a high drain-source breakdown voltage, the breakdown voltage may be reduced and punching through may occur depending on a depth of the source region. The above-described issue is caused when the source region is formed by ion implantation. When the source region is formed by ion implantation, a part of impurities may penetrate to a portion deeper than a predetermined depth. Thus, the impurity may distribute to the portion deeper than the predetermined depth. For example, when the predetermined depth (Dp) is 0.7 μm and a source region is planned to be formed to a depth of 0.7 μm, a part of impurities reach to a portion deeper than 0.7 μm although the impurity concentration gradually decreases from a predetermined concentration (Cp) at the portion deeper than 0.7 μm.